Selling items that are made to length – custom

Heyo….

We have just started setting things up for a new multisites platform (new to MarketPress….)

we need to be able to sell items using fractional quantities, or find an alternative solution, as many of the products that will be listed in some of the stores are cut to length (any length) and sold by the lineal metre.

Here is an example from a current store we are converting to WordPress

http://buybuildingsupplies.com.au/042mm-lysaght-colorbond-corrugated-roofing-p-4276.html

here the customer enters the length they need and the quantity of each in to a text field, then enters the total lineal metres in to a quantity box.

Any assistance you can offer would be greatly appreciated.